To use 20 and 50 as reference numbers, we need to be able to double and halve numbers easily.
Sometimes, such as when you halve a two-digit number and the tens digit is odd, the calculation is not so easy.
For example:
78 ÷ 2 =
To halve 78, you might halve 70 to get 35, then halve 8 to get 4, and add the answers, but there is an easier method.
Seventy-eight is 80 minus 2. Half of 80 is 40 and half of –2 is –1.
So, half of 78 is 40 minus 1, or 39.
Sometimes, such as when you halve a two-digit number and the tens digit is odd, the calculation is not so easy.
For example:
78 ÷ 2 =
To halve 78, you might halve 70 to get 35, then halve 8 to get 4, and add the answers, but there is an easier method.
Seventy-eight is 80 minus 2. Half of 80 is 40 and half of –2 is –1.
So, half of 78 is 40 minus 1, or 39.
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